The present invention is directed to an instrument capable of very accurate measurement of the magnitude of water current as well as the direction thereof. It is useful for measuring water currents in the ocean, estuaries, tidal waters, rivers and lakes. It may also be used to measure the flow of water in aqueducts, sewers, sluices and flumes. Such an instrument should be linear so that it may be employed for frequency analysis of water currents and in separating the steady and alternating components of water flow as, for example, the measurement of water flow near the surface where surface waves superpose orbital water velocities on the otherwise steady movement of water.
A water current meter may be employed in fresh, brackish or salt water, and accordingly the instrument should be insensitive to the electrical conductivity of the metered water so that its meter factor is the same in various types of water. Known water current meters are sensitive to the conductivity of the metered water whereby the meter factor is not constant in different kinds of water. For example, the zero-point or base line moves around depending on the conductivity of the metered water and this movement of the base line or zero-point generally changes with time due to the so-called electrochemical aging effects at the interface between the instrument and water.
It is desirable to not only measure the magnitude of the water current but also the direction thereof. In the past, water current meters have not successfully detected the direction of the water current where the magnitude and direction of the water current change with time.
Almost all ocean current measuring devices have employed a mechanical impeller or propeller which suffers from a number of drawbacks. The output of such meters is a non-linear function of water velocity and the output is sensitive to the speed of water movement regardless of the direction thereof. Additionally, such a construction is very much subject to marine fouling so that the meter factor can be trusted only for about 12 to 24 hours of immersion.
Fouling generally consists of the accretion of a thin layer of marine organisms on the surface of the instrument. The electrical characteristics of these organisms may be such as to alter the signal voltage thereby causing the meter to be inaccurate in operation.
In all electromagnetic velocity meters, either an alternating or a steady magnetic field may be used to develop the voltage. However, the electrical noise associated with electrode electrochemical polarization is very rich in the low frequency end of the spectrum. At zero frequency when employing a steady magnetic field, the polarization voltage is orders of magnitude larger than the flow induced voltage, and accordingly alternating magnetic induction is employed.
Alternating induction goes a long way toward solving the electropolarization problem, but it introduces a "transformer effect" noise voltage since the alternating flux threads various circuit loops in the transducer circuitry. This noise is at the same frequency as the signal produced due to the water current, although it is 90.degree. out of phase. A phase-sensitive detector can in principle reject this "transformer effect" voltage if it works perfectly, and if there are no substantial phase-shifting mechanisms in the overall transducer circuitry, including that portion of the circuitry which passes through the metered water. Unfortunately, such is not the case, and the phase-sensitive detector does not function in a perfect manner.
It is also desirable to substantially eliminate the various spurious voltages which are in phase with the signal generated by the water current and which are indistinguishable from the water current signal. These voltages have been permitted to exist in the prior art and variation in these spurious voltages with time causes a concomitant variation in the zero-point of the instrument. Accordingly, the measurements obtained by the meter are inaccurate by the amount the base line drifts from time to time.
The voltage applied to the electromagnet in an alternating induction water current meter in a practical instrument is on the order of several volts. The signal voltage generated by water motion may be on the order of 100 microvolts. If the base line is to be held steady, no spurious voltages on the order of perhaps 1 microvolt to 0.1 microvolt can be spuriously generated in the water or in the signal sensing circuitry. If only one part in ten million or one part in one hundred million of the magnet voltage is permitted to leak into the signal circuitry, the base line of the overall instrument may move around intolerably. It is therefore of fundamental importance that all voltages associated with the electromagnet be shielded so that there is substantially zero electrical admittance between the electromagnet circuitry and the water current sensing circuitry.
In electromagnetic water current meters, a very serious phaseshifting mechanism is due to the time-dependent electrochemical effects at the interface between the detecting electrodes and the water. There is, in effect, a very large capacitance due to electrochemical effects at the interface between the electrodes and the water. This capacitance coupled with the resistance of the water and/or the resistance of the electrode itself is a serious phase-shifting mechanism that shifts the "transformer effect" voltage partially so that it becomes indistinguishable from the signal voltage produced by water flow. Electromagnetic water current meters heretofore have employed a sinusoidal alternation in the magnetic induction. This electrochemical phase-shifting mechanism shifts part of the "transformer effect" voltage by 90.degree. so that it is sensed by the signal sensing circuitry which, of course, is very undesirable.
For a given phase-shifting mechanism, such as the electrochemical phase-shifting mechanism at the electrode-water interface, a sinusoid is perhaps the poorest form of alternation to be employed. In other words, the signal sensing circuitry is most sensitive to the phase-shifting mechanisms when a sinusoidal alternating magnetic induction is employed.